Molecular organic compounds are employed as active materials in a variety of applications, including organic light emitting diodes (OLEDs), photovoltaic cells, photodetectors, lasers, and thin film transistors. Typically, these thin (˜100 nm) film devices are grown by thermal evaporation in high vacuum, permitting the high degree of purity and structural control needed for reliable and efficient operation (see S. R. Forrest, Chem. Rev. 97, 1793 (1997)). However, control of film thickness uniformity and dopant concentrations over large areas needed for manufactured products can be difficult when using vacuum evaporation (see S Wolf and R. N. Tauber, Silicon Processing for the VLSI Era (Lattice, 1986)). In addition, a considerable fraction of the evaporant coats the cold walls of the deposition chamber; over time, inefficient-materials use results in a thick coating which can flake off, leading to particulate contamination of the system and substrate. The potential throughput for vacuum-evaporated organic thin film devices is low, resulting in high production costs. In past work (see M. A. Baldo, M. Deutsch, P. E. Burrows, H. Gossenberger, M. Gerstenberg, V. S. Ban, and S. R. Forrest, Adv. Mater. 10, 1505 (1998)), low-pressure organic vapor phase deposition (LP-OVPD) has been demonstrated as an alternative technique that improves control over doping, and is adaptable to rapid, particle-free, uniform deposition of organics on large-area substrates.
Organic vapor phase deposition (OVPD) is similar to hydride vapor phase epitaxy used in the growth of III-V semiconductors (see G. B. Stringfellow, Organometallic Vapor-Phase Epitaxy (Academic, London, 1989); G. H. Olsen, in GaInAsP, edited by T. P. Pearsall (Wiley, New York, 1982)). In LP-OVPD, the organic compound is thermally evaporated and then transported in a hot-walled reactor by an inert carrier gas toward a cooled substrate where condensation occurs. Flow patterns may be engineered to achieve a substrate-selective, uniform distribution of organic vapors, resulting in a very uniform coating thickness and minimized materials waste. Using atmospheric pressure OVPD, Burrows et. al. (see P. E. Burrows, S. R. Forrest, L. S. Sapochak, J. Schwartz, P. Fenter, T. Buma, V. S. Ban, and J. L. Forrest, J. Cryst. Growth 156, 91 (1995)) first synthesized a nonlinear optical organic salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate. In a variation on this method, Vaeth and Jensen (see K. M. Vaeth and K. Jensen, Appl. Phys. Lett. 71, 2091 (1997)) used nitrogen to transport vapors of an aromatic precursor, which was polymerized on the substrate to yield films of poly (s-phenylene vinylene), a light-emitting polymer. Recently, Baldo and co-workers (see M. A. Baldo, V. G. Kozlov, P. E. Burrows, S. R. Forrest, V. S. Ban, B. Koene, and M. E. Thompson, Appl. Phys. Lett. 71, 3033 (1997)) have demonstrated apparently the first LP-OVPD growth of a heterostructure OLED consisting of N,N-di-(3-methylphenyl)-N,N diphenyl-4,4-diaminobiphenyl and aluminum tris(8-hydroxyquinoline) (Alq3), as well as an optically pumped organic laser consisting of rhodamine 6G doped into Alq3. More recently, Shtein et al. have determined the physical mechanisms controlling the growth of amorphous organic thin films by the process of LP-OVPD (see M. Shtein, H. F. Gossenberger, J. B. Benziger, and S. R. Forrest, J. Appl. Phys. 89:2, 1470 (2001)).
Virtually all of the organic materials used in thin film devices have sufficiently high vapor pressures to be evaporated at temperatures below 400° C. and then transported in the vapor phase by a carrier gas such as argon or nitrogen. This allows for positioning of evaporation sources outside of the reactor tube (as in the case of metalorganic chemical vapor deposition (see S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era (Lattice, 1986); G. B. Stringfellow, Organometallic Vapor-Phase Epitaxy (Academic, London, 1989)), spatially separating the functions of evaporation and transport, thus leading to precise control over the deposition process.
As mentioned above, one type of device which employs organic thin films is a thin film field effect transistor. Thin film field effect transistors employing organic channels, for application to organic electronic circuits such as display back panels, have been made using a wide range of compounds. To date, pentacene-containing devices exhibit the highest mobilities and other favorable characteristics for thin film transistor (“TFT”) applications. See G. Horowitz, J. Mater. Chem., 9 2021 (1999). The channel layers have been deposited by several techniques including solution processing (see A. R. Brown, C. P. Jarrett, D. M. deLeeuw, M. Matters, Synth. Met. 88 37 (1997)), ultrahigh vacuum organic molecular beam deposition (“OMBD”) (see C. D. Dimitrakopoulos, A. R. Brown, A. Pomp, J. of Appl. Phys. 80 2501 (1996)), high vacuum deposition (see D. J. Gundlach, Y. Y. Lin, T. N. Jackson, S. F. Nelson, D. G. Schlom, IEEE El. Dev. Lett 18 87 (1997)), vapor phase growth of single crystals (see J. H. Schön, S. Berg, Ch. Kloc, B. Batlogg, Science 287 1022 (2000)), and finally, organic vapor phase deposition (“OVPD”) (see P. E. Burrows, S. R. Forrest, L. S. Sapochak, et al., J. of Crystal Growth 156 91 (1995); M. Shtein, H. F. Gossenberger, J. B. Benziger, S. R. Forrest, J. of Appl. Phys. 89 1470 (2001); and J. H. Schön, Ch. Kloc, B. Batlogg, Organic Electronics 1 57 (2000)).
It has been shown that the performance characteristics of organic crystalline field effect transistors, such as mobility and on/off ratio, are significantly higher in single crystals (see J. H. Schön, S. Berg, Ch. Kloc, B. Batlogg, Science 287 1022 (2000)) than in polycrystalline thin films deposited by vacuum (see Y. Y. Lin, D. J. Gundlach, T. N. Jackson, S. F. Nelson, IEEE Trans. On El. Dev. 44 1325 (1997); D. J. Gundlach, H. Klauk, C. D. Sheraw, C. C. Kuo, J. R. Huang, T. N. Jackson, International Electron Devices Meeting Technical Digest, December 1999, 111-114), flash evaporation, or solution precipitation. It has also been demonstrated that purification by train sublimation at moderate pressures (ca. 760 Torr) and in reducing atmospheres (for example, H2) results in larger and more chemically-pure organic crystals with lower trap densities than conventional vacuum train sublimation (ca. 10−5 Torr) (see Ch. Kloc, P. G. Simpkins, T. Siegrist, R. A. Laudise, J. of Crystal Growth 182 416 (1997)). Although the mobility is highest in single crystals, practical field effect transistors require the deposition of the active layers onto substrates.
Specifically, pentacene TFTs made by solution precipitation and OMBD have an effective channel hole mobility, μeff, of about 0.04 cm2/V·s at room temperature, and are typically the lowest of the described techniques. Large (ca. 0.5 cm) single crystals of compounds such as α-hexithiophene (α-6T) (see Ch. Kloc, P. G. Simpkins, T. Siegrist, R. A. Laudise, J. of Crystal Growth 182 416 (1997)), tetracene (see J. H. Schön, Ch. Kloc, A. Dodabalapur, B. Batlogg, Science 289 599 (2000)), and pentacene (see J. H. Schön, Ch. Kloc, R. A. Laudise, B. Batlogg, Phys. Rev. B 58 12952 (1998)) have been grown in inert and reducing atmospheres, with the metal contacts and gate insulators deposited onto the free-standing crystals. These devices have yielded the highest mobilities of 1.3 and 2.7 cm2/V·s at room temperature for electrons and holes, respectively (see J. H. Sch{umlaut over (0)}n, Ch. Kloc, R. A. Laudise, B. Batlogg, Phys. Rev. B 58 12952 (1998)). A more practical technique for TFT fabrication uses vacuum thermal deposition of pentacene films onto the gate insulator followed by evaporation of the source and drain contacts through a shadow mask. The resulting pentacene crystallite size is typically <1 μm, while using a shadow mask limits the minimum channel length to about 15 μm. With the channel considerably longer than the average pentacene grain size, the resulting mobility is typically between 0.1 and 0.5 cm2/V·s (see Y. Y. Lin, D. J. Gundlach, T. N. Jackson, S. F. Nelson, IEEE Trans. On El. Dev. 44 1325 (1997); and S. F. Nelson, Y. Y. Lin, D. J. Gundlach, T. N. Jackson, Appl. Phys. Lett. 72 1854 (1998)). For increased ease of integration with established technologies, and also to decrease the channel length, TFTs have been made with source and drain contacts pre-patterned by photolithography followed by vacuum deposition of the pentacene channel. However, in that case, μeff equals about 0.1 cm2/V·s, despite the shorter channel length (see S. F. Nelson, Y. Y. Lin, D. J. Gundlach, T. N. Jackson, Appl. Phys. Lett. 72 1854 (1998)).
Mobility has been reported to depend on the substrate temperature during deposition, which controls the size and connectivity of individual grains of the deposited polycrystalline thin film (see G. Horowitz and M. E. Hajlaoui, Adv. Mater. 2000 12 (1999)). Lin and co-workers have used this property to grow double-layer structures, where the first pentacene layer is grown at 70° C. to yield large grains, followed by a growth at 25° C. to fill in the intergrain gaps and increase the film connectivity (see Y. Y. Lin, D. J. Gundlach, T. N. Jackson, S. F. Nelson, IEEE Trans. On El. Dev. 44 1325 (1997)). This yielded a mobility as large as 1.5 cm2/V·s for devices where the gate insulator was pre-coated with a self-assembled monolayer (“SAM”) of octadecyltrichlorosilane (“OTS”) (see T. N. Jackson, Y. Y. Lin, D. J. Gundlach, H. Klauk, IEEE J. of Sel. Topics in Quant. Electr. 4 100 (1998)). Thin film transistors without the SAM exhibited μeff<0.5 cm2/V·s.